1. Field of the Invention
The present invention relates to new blocked polyisocyanates, to a method for their preparation and to their use for the preparation of one-component coating compositions which may be stoved at comparatively low temperatures of 130.degree. to 160.degree. C. to provide coatings with a reduced tendency to yellow upon heating.
2. Description of the Prior Art
Multilayer coatings having a glossy clear top coat based on blocked polyisocyanates and organic polyhydroxy compounds, for example, polyhydroxy polyacrylates, are gaining ever increasing importance due to their excellent properties, particularly as automotive coatings.
In this connection, butanone oxime is a conventional blocking agent for polyisocyanates which have come onto the market. Its advantages include, for example, a favorable price and a relatively low splitting temperature of approximately 150.degree. C. (without catalysis). It also reacts well with polyisocyanates and in doing so gives rise to noncrystalline products.
A blocking agent which has previously not gained the technical importance of butanone oxime is 1,2,4-triazole, which is described in DE-OS 2,812,252. Due to the tendency of polyisocyanates blocked with 1,2,4-triazole to crystallize, the preferred application of isocyanate crosslinking agents blocked with this blocking agent is in the field of powder coatings.
One-component polyurethane stoving enamels containing polyisocyanates blocked by butanone oxime have a marked tendency to yellow at high stoving temperatures or when the stoving process is of long duration.
Therefore, an object of the present invention is to provide new blocked polyisocyanates which have a reduced tendency to yellow upon heating and also have coating properties that are at least as good as those of polyisocyanates blocked with butanone oxime.
This object may be achieved with the mixed-blocked polyisocyanates according to the invention, which are described in more detail below. The blocked polyisocyanates according to the invention are blocked with butanone oxime and 1,2,4-triazole and optionally with other known blocking agents. By using the mixed blocking a distinct advance is made in achieving the objects of the present invention. A further improvement is possible through the simultaneous incorporation of specific hydrazine adducts described in more detail below.